Generally speaking, the storage capacity of a battery nowadays becomes larger and larger, and it is usually to employ a large charging current to charge the battery so as to decrease the waiting time for charging the battery. However, due to limitations of circuit elements within a conventional charger and a conventional adaptor, the conventional adaptor actually cannot supply a rated maximum charging current or even a large charging current to the battery via the conventional charger. In addition, an over large charging current may cause the circuit elements of the conventional charger thermally damaged. Accordingly, considering both of thermal protection and fast charging, it is necessary and important to provide a novel scheme for obtaining an appropriate charging current to charge the battery.